During our first ACS
funding year, we have made significant research progress. Upon completion, my
research group will possess the experimental and theoretical expertise to probe
state-specific interactions between nanoparticle plasmon resonances and molecular
electric-dipoles. The predictive understanding of plasmon-electric-dipole
interactions arising from this fundamental research program may result in
enhanced solar photo-conversion efficiency. In particular, our use of
plasmon-enhanced magneto-optical spectroscopy can impact several areas of
chemistry by providing insight into nanostructure oxidation and spin state.
Progress toward these goals in three key areas are highlighted: i)
single-particle nonlinear optical (NLO) spectroscopy for probing nanoscale
surface electromagnetic fields, ii) implementation of continuous polarization
variation (CPV) analysis to quantify nanoscale surface electric fields and
magnetization components, and iii) the assembly and characterization of
molecular-bridged nanoparticle dimers.

I. Single-particle Polarization-resolved
NLO Spectroscopy

The
first research stage involved development and characterization of a microscope
capable of second harmonic generation (SHG) measurements at the single-particle
level. Single-particle measurements are necessary because metal nanoparticles
are inherently heterogeneous. SHG, a second-order NLO technique, was chosen for
nanoscale surface field investigation because the intensity of these
measurements increases as the 4th power of local field strength.1
SHG measurements involve focusing the fundamental output of a mode-locked laser
to the sample plane using an aspheric (NA = 0.5) lens. Interactions between the
fundamental wave and the sample result in SHG, which is isolated from laser light
using a series of filters. Signal is detected by a photomultiplier tube
operated in photon-counting mode. Our home-built microscope can also collect
bright-field imaging data, allowing for spatial correlation of SHG data with
specific nanoparticle structures that have been deposited onto microscope
slides that are alpha-numerically indexed. Regions of experimental significance
are determined from the bright-field image and then subjected to structural
analysis via electron microscopy.

The extent of SHG
sensitivity to local surface fields was examined by analyzing the
depolarization ratios determined for a series of SGN dimers with different
interparticle-gap-to-particle-diameter (D/2r) values. SGN dimers were chosen as
model system because their particle diameter- and distance-dependent optical
properties are understood.2 Depolarization ratios decayed as an
exponential function of D/2r (Fig. 1), consistent with plasmon-ruler
expectations. Numerical simulations of the SGN dimer electric field reproduced
the depolarization values. To describe these data fully, it was necessary to
account for both absorption and scattering contributions to the SGN dimer
optical properties, which we detailed in a full article. The strong agreement
between calculated and experimental SHG depolarization ratios demonstrates the
capacity of single-particle SHG measurements to quantify nanoparticle surface
fields.

II. Continuous-polarization-variation-detected
SHG.

A significant
advantage of SHG measurements over emission and linear scattering techniques is
that the signal is described by the sample's nonlinear susceptibility. A
general expression describing the experimentally measured SHG intensity I(2w) as a function of the polarization of the
fundamental wave follows:

S(f) and P(f)
are the fundamental polarization states in the s and p laboratory frames; F, G,
and H are nonlinear susceptibility components. F corresponds to
electric-dipolar contributions, G to magnetic-dipolar contributions, and H to a
linear combination of the two. Therefore, SHG measurements conducted using
continuous polarization variation inform on nanoparticle surface field
symmetry.

To demonstrate our
ability to collect CPV-resolved data, SHG-detected circular dichroism
measurements were made selecting the incident polarization state as either
right or left circularly polarized and quantifying the differential SHG
response (Figure 2a). The data demonstrate a large, unambiguous difference in
the SHG signal upon switching from right to left circularly polarized incident
light, indicating a chiral plasmon field. The extent of this chirality was
determined using the SHG circular-difference ratio (SHG-CDR):

The SHG-CDR can assume values
between 0 and 2. In a study of several SGN dimers, we observed non-zero CD
values for many (but not all) structures, with a large range of experimentally
determined SHG-CDR values arising from different SGN dimers. Responses from
twenty-seven different structures (summarized in Figure 2b) demonstrate that a
high level of structure specificity can be achieved using single-particle NLO
measurements.

An important result
from this research stage is the use of colloidal nanoparticles to amplify
circularly polarized electromagnetic fields, which represents a major step
toward achieving spatially resolved single-particle magneto-optical
measurements. This research will enhance the understanding of electronic
structure of molecules near nanoparticle surfaces and provide descriptions of
interactions between plasmon modes and specific molecular states. In addition
to achieving the proposed research goals, synthesis and assembly of
nanoparticle dimers and development of advanced experimental facilities suited
for plasmon-enhanced magneto-optical spectroscopy will allow the PI to achieve
his long-term research and education objectives.

III. Molecularly-bridged SGN
dimers

Progress has also
been made in the preparation and characterization of iron porphyrin-bridged SGN
dimers. SGN dimers are formed by electrostatic interactions between the
porphyrin and negatively charged gold nanoparticles; the N-CH3+
groups of the iron porphyrin located in the interparticle gap between dimers
interacts with citrate molecules on the SGN surface. Surface-enhanced Raman
data, which exhibit prominent peaks at 660, 775, 865 and 1180 cm-1support this conclusion. These signals arise from vibrations of N-CH3+
groups positioned near the nanoparticle surface when the porphyrin binds the
metal in an edge-on geometry.3 During the next year of ACS support,
we will incorporate our NLO microscope into a superconducting magnet, allowing
for plasmon-enhanced magneto-optical imaging. Iron porphyrins, whose MCD
spectra have been described in detail, will be used as the model system.

In addition to
providing the PI with experimental infrastructure to perform fundamental
plasmon-enhanced magneto-opitcal measurements on light-harvesting
nanostructures, the research effort has provided training for three students (2
graduate, 1 undergraduate): Anne-Marie Dowgiallo prepared and characterized
nanoparticle assemblies; Jeremy Jarrett developed SHG facilities; Patrick
Herbert also made significant contributions.